My first combo was a Feral Druid + Resto Druid. Everything was pretty easy to kill. Lvling was pretty fast.
Now i have a Feral Druid + 4 Shamies, Feral Druid + 2 Priests, 2 Locks ( additionally Hunters or Mages ) combos.

The most easiest combo to box was the Shamy Team. Atm i lvl a Pala + Mage. Works pretty good too.

I used Keyclone and ISBoxer+Innerspace. For dual boxing, Keyclone is more than enough. If you wanna go 5boxing one day, i'd always prefer Isboxer+Innerspace. Its looks complicated but after some time you get used to it and its like a "wow" effect when you realize your macros work so damn good. I had some "sending keys"lag problems with keyclone with 5boxing but no Problem with Isboxer.